A large industry has developed around dried test elements used clinically to analyze serum or plasma for analytes that are a measure of a patient's health. Generally, such test elements have been designed exclusively for testing serum or plasma, in light of the fact that cells from whole blood cause interferences of various kinds. The drawback of such an approach is the necessity for centrifuging the patient sample first, to remove the unwanted blood cells. Conventionally, this has required the use of a separate centrifuge device and sample containers as well as separate operator involvement.
Such a centrifuge step has been only a minor inconvenience in those instances in which the test elements are used in hospitals or large laboratories. The reason is that such institutions have the equipment and expertise to readily perform the centrifuge separation step. However, the test elements and an appropriate analyzer have recently moved into the environment of the doctors' office. There, the need for a separate centrifuge step is a major drawback, since many doctors' offices lack the equipment and training to routinely do centrifuging prior to testing. Furthermore, the centrifuging step is time-consuming. Rapid testing is the essence of tests run in the doctors' office, in order to complete the diagnosis while the patient is still present.
Attempts have been made to convert so-called dried test elements used to analyze serum or plasma, into test elements useful also to test whole blood. Such attempts have featured the addition of a blood-cell filtering layer, above the spreading layer heretofore constituting the outermost layer. The purpose is to cause the cells to separate from the plasma, the cells being retained within the filter layer. In this way, the centrifuging step heretofore needed to obtain just serum or plasma from the whole blood, is eliminated. Examples are shown in EPO Application No. 0,159,727.
However, there are drawbacks to the approach using a blood-cell filtering layer. Chief of these is that there does not appear to be a single filter material that works for all the various test chemistries needed for the many different analytes. This may be partly due to the fact that some assays need to have reagents in the spreading layer (heretofore the outermost layer), and some have no reagents there. As a result, it has been difficult to obtain whole blood test elements for all the analytes currently tested in serum or plasma.
Therefore, prior to this invention there has been a need to provide test elements and analyzers that allow the direct testing of all analytes of whole blood, without requiring a preliminary blood cell separation step that involves separate equipment and operator involvement. Thus there is an important need, particularly in the doctors' office, is to provide a whole blood clinical analyzer for all analytes that is largely user transparent to the fact that some kind of cell-plasma separation step occurs during the process. (As used herein, "user transparent" means that the user involvement in achieving the noted step is minimal or non-existent.)